Abstract

Background

The apicomplexan parasite Cryptosporidium parvum is an emerging pathogen capable of causing illness in humans and other animals and
death in immunocompromised individuals. No effective treatment is available and the
genome sequence has recently been completed. This parasite differs from other apicomplexans
in its lack of a plastid organelle, the apicoplast. Gene transfer, either intracellular
from an endosymbiont/donor organelle or horizontal from another organism, can provide
evidence of a previous endosymbiotic relationship and/or alter the genetic repertoire
of the host organism. Given the importance of gene transfers in eukaryotic evolution
and the potential implications for chemotherapy, it is important to identify the complement
of transferred genes in Cryptosporidium.

Results

We have identified 31 genes of likely plastid/endosymbiont (n = 7) or prokaryotic
(n = 24) origin using a phylogenomic approach. The findings support the hypothesis
that Cryptosporidium evolved from a plastid-containing lineage and subsequently lost its apicoplast during
evolution. Expression analyses of candidate genes of algal and eubacterial origin
show that these genes are expressed and developmentally regulated during the life
cycle of C. parvum.

Conclusions

Cryptosporidium is the recipient of a large number of transferred genes, many of which are not shared
by other apicomplexan parasites. Genes transferred from distant phylogenetic sources,
such as eubacteria, may be potential targets for therapeutic drugs owing to their
phylogenetic distance or the lack of homologs in the host. The successful integration
and expression of the transferred genes in this genome has changed the genetic and
metabolic repertoire of the parasite.